Two of the main processes involved in semiconductor manufacturing are the etching of the semiconductor (ETCH) and material deposition on a substrate. Material deposition may be achieved by different methods, such as for example Physical Vapour Deposition (PVD), Chemical Vapour Deposition (CVD), Plasma Enhanced Chemical Vapour Deposition (PECVD) and Atomic Layer Deposition (ALD). Similar techniques are used in the fabrication of components such as flat panel displays and photovoltaic solar cells.
The present invention is applicable to various etch and deposition processes typically used during the manufacture of an Integrated Circuit (IC), a photonic device, or a solar cell. In particular, the invention is suitable for use in processes that involve the generation of radio frequency (RF) driven plasma discharge in the vicinity of the processing wafer or substrate, and can be implemented on ETCH, PVD, CVD and PECVD systems. The present invention is also applicable to plasma processing involved in the manufacture of Thin Film Transistor-Liquid Crystal Display (TFTLCD), Plasma Display Panel (PDP) and Organic Light Emitting Diode (OLED); also known as Light Emitting Polymer (LEP) or Organic Electro Luminescence (OEL). All of the above require plasma processing at some stage during manufacture.
There are a number of different etching tools that are in use by the semiconductor industry. Two commonly used etching tools or reactors for the etching process are the Capacitive Coupled Plasma (CCP) tool, and the Transformer Coupled Plasma (TCP) tool.
The principles of the etching process may be explained with reference to FIGS. 1 to 3. FIG. 1 shows a cross sectional view of a typical CCP processing tool. A vacuum chamber 10 incorporates a bottom electrode 2, on which the wafer or substrate 3 is placed, and a top electrode 7. Plasma bulk 5 is shown between the wafer 3 and the top electrode 7. A first plasma sheath 4 is located in the region between the plasma bulk 5 and the wafer 3. A second plasma sheath 6 may be located in a region between the plasma bulk 5 and the top electrode 7. A gas inlet 8 and an exhaust line 9 are also provided. The chamber also includes a bottom electrode radio frequency (RF) power supply 1.
FIG.2 shows a cross sectional view of plasma chamber 10 of a typical TCP processing tool. This processing tool incorporates substantially the same components as the CCP processing tool shown in FIG. 1, but does not include a top electrode. It also includes a second radio frequency (RF) power supply 12, an antenna 13 and a dielectric window 16. It is customary to place a matching network (not shown) between the RF power supplies 1 and 12 and the powered electrode/antenna. The purpose of the network is to match the power supply impedance, which is typically 500, to the electrodes/antenna impedance. In this embodiment, a plasma sheath 4 is located in the region between wafer 3 and plasma bulk 5.
Typical operation of such tools is explained with reference to FIG. 3, in relation to a plasma chamber 10 of a CCP tool. It involves placing a wafer or substrate 3 on the bottom electrode 2, and igniting the plasma by the radio frequency power supply 1 applying a constant amount of energy to the bottom electrode 2. A controlled gas flow of a selection of feedstock gases is also provided through gas inlet 8, which is pumped at a constant throughput into the chamber 10. The transport of etch byproducts are represented by arrows 14 through the region through the plasma sheath 4 between the bottom electrode 2 and the plasma bulk 5.
The etch process results in the removal of material from the wafer 3 by sputtering, chemical etch or reactive ion etch. The removed material is then volatised into the plasma discharge. These volatile materials are called etch-by-products 4, and, together with the feedstock gases 8, contribute to the chemistry of the plasma discharge. The etch-by-products 4 and the gases 8 are pumped away through the exhaust or pumping port 9. The etch process for a TCP tool operates in a similar fashion.
The wafer is processed by a plasma multiple times during manufacture. It has been found that plasma arcs frequently occur during the processing of a wafer. These arc events occur during or immediately following the etching or deposition steps of the processing wafer; and are related to the RF design, operating parameters, wall and shield conditions, and ageing of the processing tools.
The arc events consist of transient currents flowing between the chamber walls and/or electrodes and the surface of the wafer. Arc events can also occur between other components in the processing chamber. These are also of concern, as they can lead to damage of the substrate being processed, or can lead to damage of valuable components internal to the plasma chamber. There are various consequences that may result from the occurrence of arc events. For example, they may produce particles by sputtering material off the chamber walls, which may land on the wafer surface causing defects, or they may directly damage areas of the processing wafer.
Arcs are the result of charge build-up on a surface inside the plasma processing tool. This surface could be the plasma wafer, the chamber walls or an electrode. These surfaces, which may be either conducting or insulating, are typically coated with insulating dielectric material, being products or by-products from the processing of the wafer, resulting in an insulating dielectric layer. The charge build-up′ leads to a voltage difference build up across the plasma, the insulating layer and the wall surface. If the voltage level exceeds the breakdown strength of the wall or layers, it results in a stream of electrons, possibly with an avalanche effect, into the surface, to compensate for the charge difference. The electron population of the plasma is therefore depleted leaving behind a surplus of positive charge (ions). This electron density depletion occurs on a nano-second time scale (known as electron plasma frequency) while the relaxation time (i.e. the time taken for the plasma to return to steady state) is dominated by the ion mobility time scale and surface recharge speed rate, which is typically of the order of micro-seconds.
Hence, for typical plasma parameters of processing plasma discharges, an arc is a perturbation of the plasma state in a nano-second time scale followed by a plasma relaxation in the micro-second time scale. Multiple arc events may also occur. That is, one event may follow a second event and so on creating a longer perturbation overall, with a time scale of up to milli-seconds.
It will be appreciated that it would be advantageous to be able to monitor and detect arc events in real time. This would enable the processing tool to be taken out of the production line for a preventive maintenance (PM) if desired, once an arc event is detected. This is done to prevent further wafer scrap. If this could be achieved, it would result in reduced material costs, and the avoidance of further damage to the electronic devices under construction.
Research work for the purpose of studying arc events can be carried out with invasive IS techniques such as the electrostatic probe (also known as Langmuir probe) and/or by the placing of additional electrodes within the processing chamber to pick-up changes in the plasma state, such as the plasma potential or the electron density. However, this approach is not practical in a manufacturing environment.
Given the fast transient nature of plasma arcs, it will be appreciated that long time integrating sensors (in relation to the arc time ′scale), such as typical OES spectrometers and mono-chromators (which typically have a 100 ms integration time with 10 Hz data output speed) are not able to pick up plasma light variations generated by micro-arc events.
The arc event detection may be achieved by monitoring the RF power transmission lines from the RF power supply to the plasma source, for example as described in U.S. Pat. Nos. 6,332,961B1, 6,736,944B2 and US2008/019784A1. The principle behind this approach is based on the observation that the plasma impedance will change during the arc event, and that therefore the ratio of reflected to forward power, voltage and/or current of the driving frequency and/or harmonics may vary during the arc event. This impedance change however occurs fast enough for it not to be detected by the processing tool RF matching network system. Still, fast monitoring and processing of the RF voltage/current on the RF transmission line may reveal the occurrence of an arc event.
Mention to non-invasive optically based ′arc detection technique is made in US patent US2008/019784A1, while U.S. Pat. No. 6,332,961B1 discusses the possibility of observing high-severity arc events as a flash of light from the processing plasma discharge.